I am just a few bars of experience from hitting level 90 in Mists of Pandaria. The levelling experience in Cataclysm was the worst in WOW, even worse than vanilla I think, (maybe too much with the rose coloured glasses) but I can safely say that so far Mists is the best. Blizzard has a lot of new tools in their toolbox now that didn't exist during the initial WOW launch and this time they have really found the proper way to utilize them. With great power comes great responsibility and all that.
They have managed to recapture the feeling of freedom and exploration that existed in the game at the outset and which was quite lacking both in the new Cataclysm zones and in the old world remakes. There are random questgivers scattered here and there and it is easy to skip things and move on to something else if you want to. There are still overarching stories going on throughout the zones and some things have to be done in order but this is pretty much exactly what I asked for; a big continuous storyline with a good number of sidequests that can be done or ignored as you see fit. I can easily see myself playing through Pandaria again on an alt quite happily and I certainly did not feel that way about Cataclysm.
However, they didn't go all the way and give up on phasing and big events. There are cool story moments that only occur when you complete entire zones that wrap up everything you have been doing quite nicely, in particular the wall breach at the end of Valley of the Four Winds. They use phasing and vehicle mechanics and all kinds of NPCs and a gigantic war zone all at once and it feels very satisfying to see all the various groups I helped out throughout my adventures joining in on the event. These big events and heavy use of phasing can be fantastic when used appropriately and I think they have been used extremely well this time around.
The last thing I should comment on as far as levelling goes is the little details and useless but awesome junk that appears throughout. There are a number of different quests that award cool items at the end that are entirely fluff but are also awesome. I can create a spittoon that other players can spit into, make a golden banana, put masks on other people, and plenty of other random things. This is pure gold - the people interested in raiding or pvp or whatever can simply vendor this stuff and the collectors can have fun with all their silly trinkets and roleplaying gear.
I don't know that I will get back into WOW as seriously as I was before but I have to give credit where credit is due; aside from a couple of launch day issues Mists has been simply fantastic. Exploration, story, fluff, and interesting questing fights are all there and unless Blizzard messes up their endgame very seriously they are going to have an absolute smash hit on their hands. This time they got it right, particularly for the casual crowd.